To make streaming, downloading and storing MPEG-7 descriptions more efficient, the description can be encoded and compressed. An analysis of a number of issues relating to the delivery of MPEG-7 descriptions has involved considering the format to be used for binary encoding. Existing encoding schemes for XML, including the WBXML proposal from WAP (the Wireless Application Protocol Forum), the Millau algorithm and the XMill algorithm, have each been considered.
With WBXML, frequently used XML tags, attributes and values are assigned a fixed set of codes from a global code space. Application specific tag names, attribute names and some attribute values that are repeated throughout document instances are assigned codes from some local code spaces. WBXML preserves the structure of XML documents. The content as well as attribute values that are not defined in the Document Type Definition (DTD) can be stored in-line or in a string table. It is expected that tables of the document's code spaces are known to the particular class of applications or are transmitted with the document.
While WBXML tokenizes tags and attributes, there is no compression of the textual content. Whilst such is probably sufficient for the Wireless Markup Language (WML) documents, proposed for use under the WAP, and for which WBXML is designed, as such documents usually have limited textual content, WBXML is not considered to be a very efficient encoding format for the typical text-laden XML documents. The Millau approach extends the WBXML encoding format by compressing text using a traditional text compression algorithm. Millau also takes advantage of the schema and datatypes to enable better compression of attribute values that are of primitive datatypes.
The authors of the Xmill algorithm have presented an even more complex encoding scheme, although such was not based on WBXML. Apart from separating structure and text encoding and using type information in DTD and schema for encoding values of built-in datatypes, that scheme also:
(i) grouped elements of the same or related types into containers (to increase redundancy),
(ii) compressed each container separately using a different compressor,
(iii) allowed atomic compressors to be combined into more complex ones, and
(iv) allowed the use of new specialized compressors for highly specialized datatypes.
Nevertheless, existing encoding schemes are only designed for compression. They do not support the streaming of XML documents. In addition, elements still cannot be located efficiently using the XPath/XPointer addressing scheme and a document cannot be encoded incrementally as it is being constructed.